Stable lead alkyl compositions and a method for preparing the same



United States Patent Ofiice 3,004,997 Patented Oct. 17., 3961 3,004,997 STABLE LEAD ALKYL COD [POSITIONS AND A METHGD FOR PREPARING THE SAL [E Hymin Shapiro, Baton Rouge, La., and Herbert R. Neal,

Farmington, Mich., assignors to Ethyl Corporation,

New York, N.Y., a corporation of Delaware No Drawing. Filed Dec. 8, 1959, Ser. No. 858,041 3 Claims. (Cl. 260-437) This invention relates to alkyllead compositions which are stable at temperatures above 100 C. It also relates to methods for inhibiting the thermal decomposition of alkyllead compounds when subjected to temperatures above 100 C., at which temperature thermal decomposition becomes appreciable.

Generally this invention contemplates inhibiting the thermal decomposition of alkyllead compounds in which at least one valence of the lead is satisfied by an alkyl radical.

More specifically, this invention is concerned with an improved process for separating alkyllead compounds from the reaction products accompanying their synthesis. It is also applicable to a method for inhibiting thermal decomposition of an alkyllead product during its purification and blending with other products in making commercial antiknock fluids. It is applicable to minimizing the possibility of thermal decomposition during storage or transportation of an alkyllead product. It is especially applicable to preventing thermal decomposition of undiluted alkyllead compounds where the likihood of thermal decomposition is more of a problem.

As is well known, tetraalkyllead antiknock compounds generally are produced by reacting a sodium-lead alloy with an alkyl halide. Due to recent marked improvements in the technology of alkyllead manufacture, thermal instability of alkyllead compounds during synthesis is no longer a problem. However, the tetraalkyllead compound so produced is in admixture with various reaction by-products from which it must be separated. Separation is effected by steam or vacuum distillation with subsequent purification of the tetraalkyllead distillate. Due to the toxic and unstable nature of tetraalkyllead antiknock compounds, these distillation and purification operations are subject to many difiiculties.

In these distillation and purification operations meticulous temperature control and exact safety measures are of paramount importance. The rate of decomposition of the alkyllead compound increases rapidly with small rises in temperature above the temperature where thermal decomposition becomes appreciable. For example, decomposition of tetraethyllead occurs at the rate of approximately 2 percent per hour at a temperature of 100 C., which is the customary temperature used in separating tetraethyllead from the reaction products accompanying its synthesis. At temperature above 100 C. the decomposition rate increases logarithmically so that a point as soon reached where external heat is no longer required and decomposition becomes self-propagating.

Such likelihood of excessive decomposition is present also during blending, handling, storage, and transportation. Prior to diluting the alkyllead concentrate with scavengers, gasoline or other materials, the alkyllead compound remains a concentrate and the problem of excessive thermal decomposition exists, even though the temperature is maintained normally well below that of decomposition. For example, in the purification step wherein the tetraethyllead concentrate is washed and blown with air at atmospheric temperature to remove impurities, a sudden increase in temperature may occur dut to the oxidation of triethylbismuth which is present as an impurity. Also pumps used in handling tetraethyllead occasionally freeze and the fiiction developed may cause a local overheating to a temperature above the temperature of decomposition of the tetraethyllead. Faulty wiring, leaks onto steam pipes, and other accidental causes also may produce local overheating with resulting dangerous decomposition.

It is seen therefore that in those operations where an alkyllead compound is in the undiluted or concentrated stateviz. separation, purification, blending, transportation, and storage-the likelihood of excessive thermal decomposition must be provided for and effectively combatted.

An object of this invention is to stabilize alkyllead compounds against thermal decomposition during one or more of the following operations: separation, purification, blending, transportation, and storage.

The above and other objects of this invention are accomplished by incorporating with alkyllead compounds a relatively small quantity of a material which has the property of inhibiting alkyllead thermal decomposition. The foregoing objects are also accomplished by conducting one or more of the foregoing operations in the presence of such a material. The materials which have been found to possess this property are referred to hereinafter as thermal stabilizers.

The thermal stabilizers of this invention are saturated aliphatic aldehydes. Generally speaking, the alkyl group of these thermal stabilizers contains from 2, and preferably from 3, up to about 16 carbon atoms. These thermal stabilizers when used in amount varying from about 1.75 to about 10 percent by weight of the lead alkyl product are eifective in substantially retarding or preventing thermal decomposition at temperatures above C. for extended periods of time.

Another important feature of this invention is the fact that the foregoing thermal stabilizers are, in general, stable to heat, light, exposure to air, and other conditions to which alkyllead compounds may be subjected during their separation, purification, blending, transportation, and storage. Thus, for example, the thermal stabilizers of this invention have no tendency to react to form gums or other obnoxious reaction products in the alkyllead composition.

Another important feature of this invention is that the foregoing thermal stabilizers are relatively inexpensive and easily made.

Typical thermal stabilizers of this invention are propionaldehyde; butyraldehyde; isobutyraldehyde; Valeraldehyde; capronaldehyde; heptaldehyde; octaldehyde; nonaldehyde; decaldehyde; dodecyl aldehyde; cetyl aldehyde; and the like. Acetaldehyde can also be used in blending, storage and transportation operations but is less preferably used in the separation operations because of its lower boiling point.

The chief thermal decomposition products of alkyllead compounds are lead metal and hydrocarbon gas. Hence, a very good index of alkyllead thermal decomposition is liberation of this gas.

To illustrate the effectiveness of this invention, direct comparisons were made between the decomposition characteristics of unstabilizcd and stabilized tetraethyllead. A thermostatically controlled hot oil bath was fitted with a stirrer, thermometer, and a holder for a small reaction tube. A 100 cc. gas buret beside the bath, and equipped with a water-containing levelling bottle, was connected by means of rubber tubing with the reaction tube after the desired sample was introduced into this tube. After the bath was brought to a steady temperature of C., the sample-containing tube was quickly immersed in the bath and clamped with the levelling bottle adjusted to hold the gas buret in place at a zero reading. Then measured was the time during which the sample was held at 160 C. without pronounced thermal decomposition and consequence gas evolution occurring. Thus, the longer the time, the more thermally stable was the alkyllead composition.

With pure tetraethyllead used in 1 ml. amounts, pronounced thermal deterioration occurred within 1 minute as evidenced by rapid gas evolution. In fact, the decomposition of unstabilized tetraethyllead will normally become uncontrollable if it is heated, whether rapidly or slowly, to even 130 C., unless it is possible to very rapidly cool it down to about 100 C. or below.

However, when to the same amount of tetraethyllead there was previously added 2 percent by weight of heptaldehyde no pronounced deterioration occurred at 160 C. for over 40 minutes. The same order of effectiveness subsists when repeating this experiment using in one instance 3 percent by weight undecyl aldehyde, and in another instance 6 percent by weight of propionaldehyde. In fact, comparable effectiveness is exhibited by all of the thermal stabilizers of this invention.

An important facet of this invention is the discovery that the thermal stabilizers must consist of an aldehyde grouping attached to a saturated aliphatic 'open chain (i.e. an alkyl group). The great importance of these structural criteria is illustrated by the fact that if the aldehyde groupingis attached to an aromatic ring instead of to an alkyl group, the resultant material has little or no effectiveness as a thermal stabilizer. By Way of example, 2 percent by weight of acetophenone failed to produce any significant inhibition of tetraethyllead decomposition at 160 C. when tested in the manner described above. In fact, saturated aliphatic open chain hydrocarbons themselves are totally ineffective.

The above-described beneficial behavior of the thermal stabilizers of this invention also takes place with other alkyllead compounds such as triethyllead bromide and tetrapropyllead. In fact, these compounds when stabilized can be boiled and distilled at atmospheric pressure.

This invention is adapted to the stabilization of tetraethyllead and other alkyllead compounds at various stages after they have been formed and the diluents or excess alkyl halide have been dischargedfrom the autoclave. For example, one of the above thermal stabilizers may be added in appropriate quantity to the alkyllead reaction concentrate just before the separation step which is conducted at a temperature close to the temperature where hazardous run-away decomposition is particularly prevalent. By adding one of the above thermal stabilizers to the reaction concentrate just prior to distillation, the danger arising from unexpected temperature increases is substantially eliminated.

Most preferably the above thermal stabilizers are employed to stabilize the alkyllead compound both in storage and in shipping and especially to stabilize any alkyllead concentrate, i.e., compositions containing at least percent by weight of alkyllead compound. If elevated temperature conditions are likely to be encountered, the addition of a small amount of thermal stabilizer to the alkyllead compound will economically and satisfactorily eliminate most of the hazard involved. While meticulous temperature control and exacting safety measures have been successful in reducing to a minimum the hazards of processing and handling of tetraethyllead, the use of this invention provides a much greater factor of safety. Furthermore, waste of the alkyllead product due to decomposition is considerably minimized through the use of this invention.

This invention is useful in stabilizing alkyllead compounds in which at least one valence of the lead is satisfied by an alkyl radical. For example tetraethyllead, tetramethyllead, tetrapropyllead, dimethyldiethyllead, triethylphenyllead, and triethyllead bromide .can be successfully stabilized against thermal decomposition by incorporating thereina relatively small quantity of one of the thermal stabilizers ofthis invention.

What is claimed is: V

1. A method inhibiting the decomposition of an essentially pure alkyllead compound at temperature from about C. to about C., which comprises incorporating with said compound a small amount of a saturated aliphatic fldehyde, the alkyl group of which contains from 2 to about 16 carbon atoms; said amount being sutficient to inhibit decomposition and being in the range of from about 1.75 to about 10 percent by weight based upon Weight of said compound.

2. An essentially pure alkyllead compound containing, in amount sutficient to inhibit thermal deterioration thereof at temperatures from about 100 C. to about 160 C., a saturated aliphatic aldehyde, the alkyl group of which contains from 2 to about 16 carbon atoms; said amount being in the range'of from about 1.75 to about 1 0 percent by weight based upon the weight of said compound.

3. Essentially pure tetraethyllead containing a small amount of heptaldehyde suificient to inhibit thermal deterioration of the tetraethyllead at temperatures from about 100 C. to about 160 C.; said amount being in the range of from about 1.75 to about 10 percent by weight based upon the weight of the tetraethyllead.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD INHIBITING THE DECOMPOSITION OF AN ESSENTIALLY PURE ALKYLLEAD COMPOUND AT TEMPERATURE FROM ABOUT 100*C. TO ABOUT 160*C., WHICH COMPRISES INCORPORATING WITH SAID COMPOUND A SMALL AMOUNT OF A SATURATED ALIPHATIC ALDEHYDE, THE ALKYL GROUP OF WHICH CONTAINS FROM 2 TO ABOUT 16 CARBON ATOMS, SAID AMOUNT BEING SUFFICIENT TO INHIBIT DECOMPOSITION AND BEING IN THE RANGE OF FROM ABOUT 1.75 TO ABOUT 10 PERCENT BY WEIGHT BASED UPON WEIGHT OF SAID COMPOUND. 